A new strain of Bacillus circulans isolated from a larva of Culex quinquefasciatus showed larvicidal activity on 3 mosquitoes of medical importance. Compared to Bacillus sphaericus strain 2362, this B. circulans isolate proved less toxic to Cx. quinquefasciatus and Anopheles gambiae but was 107 times more toxic to Aedes aegypti. Moreover, in comparison to other studies, B. circulans was at least as pathogenic as B. thuringiensis var. israelensis in Ae. aegypti. The tests have showed that the toxicity of the bacterial culture of B. circulans resulted from its spores and not from the insecticidal effect of chitinases or exotoxins.